The long-term objective guiding the current proposal is a quantitative understanding of the fundamental molecular mechanisms driving apomyoglobin folding. Detailed structural and dynamical characterization of equilibrium and kinetic folding intermediates is a critical step toward the prediction of protein structures from amino acid sequence and the rational development of drugs. This research will address several important questions surrounding the equilibrium apomyoglobin folding pathway. Initial studies will focus on characterizing the topological heterogeneity and extent of native-like helical packing in the molten globule structural ensemble through computational modeling of site-specific spin label derived distance constraints. Subsequent work will focus on utilizing relaxation dispersion NMR measurements to monitor microsecond to millisecond timescale dynamics along the equilibrium unfolding pathway with emphasis on resolving early collapse events in the acid-unfolded state and cooperatively formed folding nuclei preceding molten globule formation. Once the relaxation dispersion kinetics have been thoroughly explored in the wild type protein, more qualitative measurements will be made on apomyoglobin mutants which exhibit altered core packing to answer fundamental questions about plasticity and adaptability in the collapse process.